It's not a good one to use as your Primary key for use in relationship links to other tables of data. It may however, need to be made to support special labeling needs--possibly in support of existing procedures already practiced by your end users. In such cases, you can set up this field as a data field so that it can be used in searches, sorts, reports, etc.--just not as a match field in relationships.
You can Divide this serial number into two fields, one with the category value and one with the sequence number. You can then set up a self join relationship that matches by category field so that the following calculation can be used to assign the next sequence number value:
Max ( SelfJoinTableOccurrence::SequenceNumber ) + 1